Directional characteristics of electroacoustic transducers and method for utilizing the same



2,830,283 O-ACOUSTIC FOR April 8, 1958 F. MASSA DIRECTIONAL CHARACTERISTICS OF ELECTR TRANSDUCERS AND METHOD UTILIZING THE Filed April 18, 1950 SAME 3nnentor DIRECTIONAL CHARACTERISTICS OF ELECTRO- ACOUSTIC TRANSDUCERS AND METHOD FOR 4 THE SAME Frank Massa, Cleveland Heights, Ohio Application April s, 1950, Serial No. 156,668

S Claims. c1. s40-s My invention is concerned with directional sound generating and receiving systems and, more particularly, with means for obtaining directional patterns that have predetermined spatial characteristics. For example, it is of the transducer in Fig.2.

well-known that in conventional direction electro-acous- 1 tic transducers, such as, for example, vibrating circular pistons or rings whose diameters are large compared with the wavelength of the generated or received sound vibration frequency, that the directional characteristic of the transducer consists of a main lobe whose maximum is'on the normal axis of the transducer, and a numberof secondary lobes spaced away from the normal axis of the piston or ring. In some applications, the large variation in sensitivity which results in such a conventional transducer as a function of angle from the normal; axis is very objectionable. A. directional pattern having a gradual change in sensitivity with angle instead of. the. violent variation of maximum and minimum sensitivities as, occur in the above cited conventional examplesis, in many instances, very desirable. In previous attempts t to minimizethese objectionable variations in'the directionalcharacteristics of trans'- ducers, it hasb'een thelpracticejto design vibrating structures in which diiierent. portions of the vibratingsurface of the transducer operate with varying amplitudes or .phase. Besides being very costly to manufacture such composite structures, the variation between the ,Patented Apr. 8,- 1958 Fig. 1 is a polar diagram showing the directional radiation characteristic of, a transducer at two discrete frequencies and also the resultant directional radiation characteristic of the transducer when it is operated in a circuit which utilizes the basic principle of my invention.

Fig. 2 is a schematic representation of an echo-ranging system in which a conventional transducer is employed as a receiver and frequency selective circuit'elements are employed in the output circuit of the transducer which causes the elimination ofthe sharp vari-' ations in the directional radiation pattern of the transducer in accordance with the teachings of my invention.

Fig. 3 is a graph illustrating the change which is caused in the frequency response characteristic of the transducerillustrated in Fig. 2 due to the tuned circuit elements shown incorporated in the output connection Referring more particularly to Fig. 1, there are shown three curves,labelled'1, 2, and 3,'each representing the directional chracteristic of the transducer as a function of angle aunder difierent conditions. The curve 1 has actually been computed for a plane piston surface operating at a frequency f in which the wavelength of sound in the medium is approximately three-fourths the diameter of the piston. The curve 2 shows the directional characteristic of the same piston as a function of angle for a higher frequency i in which the Wavelength of sound in the medium is approximately one-half, the

diameter of the piston. The solid curve S'shows the resultant, directional characteristic of the same piston when it simultaneously is allowed to respond to the two .frequencies f +f The axis labelled 0 represents a line normal to the surface of thepiston which comprises the active face of the transducer. The curves in Fig. 1 show the relative sensitivity of the transducer in decl ibels as a function of the angle 0 removed from the maximum and minimum sensitivitieswithangle is usual-- lyonly reduced by this procedure and not actually eliminated.

It'is the major object ofmy'i'nvention to provide dire'ctional patterns having smooth, gradually varying sen-;

sitivities as a functionof angle in transducersthat are basically very simple structures.

A further object of my invention is to tailor make directional patterns] of an electro-acoustic transducer'by thesimple expedient of; designing a given frequency selectivity into the electrical circuit associated 'with the unit; A still further object of myinvention is to improve the directionaI patterns of electro-aconstic transducers without changing'the mechanical design of the basic transducer element; a

Another object of my invention is to provide a method for the directionalsendingandreceiving of sound energy in which the radiation 'patternin the medium is free of thefviolent fluctuations'in angular sensitivities which fareinherently prescntin the basic transducers employed in the. system when operating at singlefrequencies.

@The novel features that I consider characteristic of i a invention are set. forth with particularity in the ap- ,pe nd ed claims. The inventi'omitself, however, both as to its organization and method of operation, 'as well as advantages thereof,fwill best. be understood from the tgllowing";.description of several embodiments thereof, the accompanying draw when read? in connectio'nlwith ings, in which:

metry drawn normal to the plane containing the ring.-

For the case of the ring .type transducer, the relative magnitudes, as a-functionof angle 0, on the directional characteristics would be actually ditlerent than shown in Fig. 1, as will'be. indicated .below, but the principle. of achieving an improved directional pattern by the 211 plication of the teachings of my. invention arethe. same.

The directional characteristics illustrated in Fig; 1. could also represent the directional patterns of. a long line,

in which case the relative magnitudes would be again different than either those of the piston or the ring but,'again, the principle-of the application of the teachings of my inventionremain the same. A line type: transducer is well known to those skilled in the art and.

US. Patent 2,613,261 and further described in the;

paragraph beginning on line 39 of column 7 in saidpatent. The'patterns indicated in Fig; 1, if illustrative of a line type transducer, would be the characteristics in a plane containing the line of which the axis labelled 0' would be the perpendicular bisector of the line." In this case, however, as already mentioned, the relativemag nitudes of the response-as a function ofangle wonlcl be dilferent' th'an the relative response as a function of angle for-either the case' 'of theipis'ton surface'or 'thef' ring surface, as, will be shown-by theequations below.- In the case "of '-eitlier"the piston or the ringtypetrails' Although the curves shown 'inches at the operating frequency f ducer," the curves shown in Fig. 1 would be symmetrical I about the axis labelled while for the case of a line type transducer, the curves would be symmetrical about axis labelled 90 which contains the normal axis of the line. e a

' Still referring to Fig. l, the curve 1 indicates the directional characteristic of a plane piston operating at a frequency h. The ratio of the response P at an angle 6 to the response P on the normal axis of the piston is given by the well-known expression:

where:

P =response at angle 0 P response along the normal axis (0:0)

J Q=Bessel function of first order- D=diarneter of the piston in inches X =wavelength of sound in the medium in inches, corresponding to the frequency 1;

6=angle removed from normal axis in degrees where:

D=diameter of the ring in inches J =Bessel function of zero order and the other'symbols are the same as in Equation 1.

For the case of a thin line, the ratio of the response P at an angle 6 to the response P along the perpendicular bisector of the line would give the following shape to curve 1: I

where L=length of line in inches and the other symbols are the same as in Equation 1.

The curve 2 in Fig. 1 is the directional pattern for the same piston for which curve 1 was computed except that the frequency has been increased to f This curve is given by Equation 1 above by substituting A for A in which A, corresponds to the wavelength of the sound in Similarly, the corresponding curve 2 for a ring or a line may be obtained by substituting x for A in Equations 2 and 3 above.

The curve 3 in Fig. 1 is the sum of curves 1 and 2 and represents the directional characteristicof a simple circular flat piston when-operating simultaneously and at equal sensitivity at both frequencies and f By allowing the transducer to respond simultaneously to the two separate frequencies, I am able to achieve a smooth directional characteristic as illustrated by curve 3 in Fig. 1. The basic reason why this curve is free from sharp null points is that I have added the curves 1 and 2 vectorially. This is permissible only because I have employed two separate different frequencies for the component curves 1 and 2 in the determination of, the overall characteristic 3 and it is this principle of combining two different frequency directional characteristics which is the basic part of my present invention.

In previous attempts at improving the directional characteristcis of transducers, two curves similar to 1 and 2 of Fig. 1 where obtained by using a composite piston in which different portions operated at different amplitudes. These curves, being of the same frequency, how-- ever, had to be combined algebraically, which resulted in the retention of sharp nulls in the overall characteristic hs f mp e; aUe s?! 74 42 -96 i sue? to Frank Massa on September 9, 1947. The algebraic addition of two directional patterns such as curves 1 and 2 when they would be of the same frequency must take cognizance of the alternate phase reversals of the successive secondary lobes in each of the curves. This would mean that at the point at which the main lobe of curve 1 crosses the secondary lobe of curve 2, the two response components would be of equal magnitude and of opposite phase (were the curves representing the same frequency as in the case of a composite piston surface operating at different amplitudes) and thus the combined sensitivity would be zero. For the actual case in this present invention, however, curves 1 and 2 represent different frequencies so that phase relations have no significance and the resultant curve 3 is always the root mean square sum of the individual components. Thus, by employing this new concept, I am able to achieve a smooth directional characteristic without sharp nulls as is actually shown by the curve 3 in Fig. 1.

One large advantage of my invention is that existing transducers may be made to yield smooth directional characteristics without change in design of the transducer structure but by making a relatively simple circuit modification in which the circuit is selectively sensitive to two separated frequencies which are desired to be reproduced by the transducer. Figs. 2 and 3 illustrate my new method for the directional sending or receiving of a sound wave through an elastic medium for the purpose of improving the directional response characteristic of the transducer. In Fig. 2, T represents a conventional transducer which may be a flat circular piston which is facing down toward the bottom of the sheet in the drawing. In Fig. 2, S represents a sound generator which may be electrically, mechanically, or chemically excited and which generates more than a single frequency of vibration. The source S may generate a band of frequencies or only the frequencies f, and f which are going to be selectively received by the electrical circuit connected to the transducer. When the arrangement schematically illustrated in Fig. 2 is used as an echo-ranging system, for example, a burst or a pulse of sound of short duration is caused to be discharged from the source S. The sound is reflected from the distant surface 4 and the returned echo is picked up by the transducer'T. The voltage V generated by the transducer is passed through the series tuned circuits C L and C L connected as shown in Fig. 2 and a voltage V appears across the output resistance R. a

A graph showing the circuit response and its selectivity to the transducer generated voltage is plotted in Fig. 3. The abscissa in Fig. 3 represents frequency and the ordinate indicates magnitude of the response. At the transducer terminals, a voltage V is generated by the receivedecho which is uniformly flat with respect to frequency. This assumes that the source S has generated sound having a uniform spectral distribution of energy and also that the receiving transducer T is uniformly sensitive over the frequency spectrum. The effect of the two series tuned circuits L 0 and L C is to cause only the frequencies f and f representing their respective series resonant frequencies, to be present in the output voltage V If the frequency is chosen to correspond to the directional characteristic of the transducer T as is shown by curve 1 in Fig. 1 and if the frequency f is chosen to correspond to the characteristic shown by curve 2 then the end result of the method illustrated in Fig. 2 is to cause the transducer T to have the effective directional characteristic as illustrated by curve 3 in Fig. l. The advantage of not having any sharp nulls in the directional'characteristic of the transducer by employing the teachings of this invention is obvious in that there will be no blind spots in which an echo may fail to be received such as might occur when employing conventional transducers in conventional prior art receiving circuits. I I

1 may be excited to generate only the frequencies f and f;.

In either case, after the sound pulse is emitted and the ,circuit is switched to receiving position, the reflected signal picked up by the transducer will be applied to the selective circuit schematically shown in Fig. 2 to cause the results already described for the eifective smoothing of the directional characteristic of the receiving system.

I have found that optimum results can be obtained in correcting the directional characteristic of simple transducers, in accordance with the teachings of my invention, if the two selected frequencies 3 and f are chosen so that the lower frequency f lies in therange from to 50% below the frequency f If single frequencies are not used, the individual frequency regions are each preferably less than A octave in band width.

Although I have chosen only a few specific examples to illustrate the basic principles of my invention, it will be obvious to those skilled in the art that numerous departures may be made from the details shown, and I, therefore, desire that my invention shall not be limited except insofar as is made necessary-by the prior art and by the spirit of the appended claims.

I claim as my invention:

4. The invention set forth in claim 1, characterized in that said electro-acoustic transducer is of the line type and further characterized in that the length of the line is greater than the wavelength of the sound at the highest frequency which is selectively passed by said electric circuit.

5. In combination, in a directional system for the reproduction of sound, an electro-acoustic transducer, said transducer being characterized in that its directional response characteristic is difierent for each of two separated frequency regions 'of operation, an electric circuit connected to said transducer, two tuned circuit elements asso- 1. In combination, in a directional system for the V reproduction of sound waves, an electro-acoustic transducer for the conversion of sound vibrations to electric signals, said transducer being characterized in that its directional response characteristic is different for each of two separated frequency regions of operation, an electric circuit connected to said transducer, frequency selective means associated with said electric circuit, said frequency selective means .being characterized in that two separated frequency regions are selectively passed by said electric circuit, and means for combining signals from each of the said two separated frequency regions whereby said combined signal from said transducer results in a directional response characteristic which is a combination of the two different directional response characteristics displayed by the transducer for each of the two separated frequency regions of operation.

2. The invention set forth in claim 1, further characterized in that the lower of said two separated frequency regions lies in the range from 10% to below the higher of said frequency regions.

3. The invention set forth in claim 1, further characterized in that the active portion of said electro acoustic transducer comprises a plane surface having at least one dimension greater than the wavelength of sound at the highest frequency which is selectively passed by said electric circuit.

ciated with said electric circuit, each tuned circuit element tuned to a difierent frequency, said tuned circuit elements being adapted to permit said electric circuit to be selectively sensitive to each of said different tuned frequencies, and. means for combining signals from each of the said two different frequencies whereby said combined signal from said transducer results in a directional response characteristic which is acombination of the" two diiferentdirectional response characteristics displayed by the transducer for each of the two different frequencies.

6. The invention set forth in claim 5, further characterized in that the lower of said two, separated frequency regions lies in the rangefrorn 10% to 50% below the higher of said frequency regions.

7. The invention set forth in claim 5, further characterized in that the active portion of said electro-acoustic transducer comprises a plane surface having at least one dimension greater than the wavelength of sound at the highest frequency which is selectively passed by said electric circuit.

8. The invention set forth in claim 5, characterized in that said electro-acoustic transducer is of the line type and further characterized in that the length of the line is greater than the wavelength of the sound at the highest frequency which is selectively passed by said electric circuit.

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